There are two possible reasons.
The first reason is that you are in demo mode and you try to open a model created on another PC which exceeds the number of nodes, while the model created on your PC is lower than the limitation. In demo mode the limitation is given by 500 nodes per slices with a maximum number of 5 slices.
The other reason is that you do not have a cooperate license, but you try to open two models at the same time.